HIV-1 isolates were recovered from biopsy tissues from the small bowel, colon, and rectum of 10 infected individuals with severe diarrhea. In general, the bowel strains grew well in primary macrophage and lymphocyte cultures, not in T or B cell lines. They induced cytopathic effects such as syncytia formation and cell killing in peripheral blood mononuclear cells and were usually sensitive to serum neutralization. Several of these isolates were able to infect bowel epithelial cell lines, but this characteristic was also observed with blood-derived strains. Differences could be identified in 3 of 6 cases of paired bowel and blood isolates from the same individual. When compared to blood-derived isolates, the bowel strains exhibited a relative inability to grow in established cell lines, a reduced ability to induce cytopathology in infected cells, and a greater sensitivity to serum neutralization. Thus, although distinct characteristics of bowel-derived HIV-1 strains were not found, certain biological and serological properties might differentiate these viruses from those isolated from other tissues.